1. Field of the Invention
The invention relates in general to a fabrication of a MOSFET (metal oxide semiconductor field effect transistor) device, and more particularly to a reverse T-gate MOSFET device.
2. Description of the Related Art
When the semiconductor device becomes smaller, the operating rate is raised and the channel length is shortened. The channel length of the MOSFET can not be shortened limitlessly. When the channel length of the MOSFET is shortened to a certain degree, the short channel length causes many problems. This is called a short channel effect. For example, if the threshold voltage Vt decreases, then it becomes difficult to apply a proper gate voltage Vg. When the channel length of the MOSFET is shortened, and the voltage applied to the gate is the same, the transverse electric field will increase. By the accelerating of the electric field, the electrons in the channel have a higher energy. In particular, the electrons in the region close to the junction of the channel and the drain possess much more energy and are called "hot electrons". When the hot electrons bump the electrons in the valence band, the electrons in the valence band go to the conduction band. Then, the electron-hole pair generates the "carrier multiplication" effect. If the "carrier multiplication" effect is serious, it will result in electrical breakdown.
In addition, the increase of the ICs' integrity will cause the junction depth of the source and the drain to be shortened. As a result, the distribution and profile of the semiconductor is difficult to control accurately. The channel effect of ions implantation causes depth control to be difficult. The channel effect of ions implantation will cause an over-implanting depth and result in a damage of the device.
In order to increase the ICs' integrity and to keep a better product quality of a 0.25 .mu.m or lower channel, the resistance of the polysilicon should be lowered to increase the speed of the device. Besides, the junction depth should be lowered to prevent the device from having the channel effect. FIGS. 1A to 1F are cross-sectional views of conventional schematic processes of forming a MOSFET device.
Referring to FIG. 1A, a device includes a substrate 100, a gate oxide layer 102 and a gate 104, wherein the gate oxide layer 102 is formed on the substrate 100, and the gate 104 is formed on the gate oxide layer 102. A liner oxide layer 106 is formed on the device. Then, the substrate is implanted to form a lightly doped region 108.
Referring to FIG. 1B, a dielectric layer 110 is formed on the device. Then, the dielectric layer 110 and the liner oxide layer 106 are etched to become a liner oxide layer 106a and a spacer 110a. Next, the substrate 100 is implanted to form a heavily doped region 112. As shown in FIG. 1C, the lightly doped region 108 and the heavily doped region 112 form a lightly doped drain (LDD) structure.
Referring to FIG. 1D, a titanium layer 114 is formed on the device. Then, a self-aligned silicide process is performed on the contact surface of the titanium layer 114 and the gate 104 with the substrate 100 by annealing to form a titanium silicide layer 116 as shown in FIG. 1E. Referring to FIG. 1F, the remainder of the titanium layer 114 is removed to complete the fabrication of the self-aligned silicide.
In the conventional processes, the distance between the lightly doped region and the titanium silicide layer is small. As a result, current leakage occurs. The current leakage will be especially serious when a voltage is applied to the gate. This is the so-called gate-induced dual leakage. In order to generate a better drain current I.sub.D, the lightly doped region can be further implanted to form a heavily doped region. The current technology of implantation is unable to implant the ions in the correct region. If the heavily doped region is formed, the large junction capacitance in the lightly doped region and the channel will cause the short channel effect. In addition, the small contact area of the silicide on the gate will have a low operating rate and a hot carrier effect.